A system for the accurate localization and fixation of the prostate during the planning and treatment of prostate caner using radiation therapy has been developed. The visualization of the prostate gland is achieved using the technique of transrectal ultrasonography. During the planning phase of the treatment the precise position of the prostate gland, as defined by the transrectal ultrasound image, relative to proximal anatomical structures, such as the rectum and bladder as defined by diagnostic x-ray images, is determined. Following the planning phase, the accurate localization in real time of the prostate relative to the radiation treatment field is also determined. Both of these quantitive localizations are achieved by the unambiguous transformation of the ultrasound image data to the space of the diagnostic images during the planning phase and to the space of the radiation field during the treatment phase. The exact position of the ultrasound image data relative to the radiation field is achieved by the development of a couch mount for the transrectal probe. Real time imaging software has been developed to carry out the image transformations and to aid in visualization process during the planning and treatment phases of radiation therapy. PROPOSED COMMERCIAL APPLICATION: The outcome of this research could be of immediate benefit in treatment of prostate cancer using higher dose radiation with fewer toxicities and less morbidity, and the potential for improved local control. Successful implementation could result in a change in the standards for prostate radiation therapy potentially affecting patients. Commercial potential is very significant.